Nonthermal refining type high tension steel exhibiting excellent cold-work-ability

ABSTRACT

A NONTHERMAL REFINING TYPE HIGH TENSION STEEL SUBSTANTIALLY CONSISTING OF 0.05 TO 0.15% C AND COMPLEX ADDITION OF 0.01 TO 0.10% NB, 0.03 TO 0.15% TI AND 0.05 TO 0.15% ZR, UNAVOIDABLE IMPURITIES AND REMAINDER FE, EXHIBITS HIGH TENSILE STRENGTH OF MORE THAN 60 KG./MM.2 OR YIELD STRESS OF MORE THAN 45 KG./MM.2 AND EXCELLENT COLDWORKABILITY.

March 5, 1974 TETSUO YAMAGUCHI ETAL 3,795,505

NONTHERMAL REFINING TYPE HIGH TENSIONv STEEL V EXHIBITING EXCELLENT COLD-WORKABILITY Filed April 19. 1972 3 Sheets-Sheet 1 March 5, 1974 .TETSUQ YAMAGUCHl ETAL 3,795,506

NONTHERMAL REFINING TYPE HIGH TENSION STEEL EXHIBITING EXCELLENT COLD-WORKABILITY Filed April 19. 1972 3 Sheets-Sheet 2 March 5, 1974 TETSUO YAMAGUCHI ETAL 3,795,506

NQNTHERMAL REFINING TYPE HIGH TENSION STEEL EXHIBITING EXCELLENT COLD-WORKABILITY Filed April 19. 1972 3 Sheets-Sheet 3 United States Patent O rm. c1. C22c 57/00, 39/00 US. Cl. 75-123 4 Claims ABSTRACT OF THE DISCLOSURE A nonthermal refining type high tension steel substantially consisting of 0.05 to 0.15% C and complex addition of 0.01 to 0.10% Nb, 0.03 to 0.15% Ti and 0.05 to 0.15% Zr, unavoidable impurities and remainder Fe, eX- hibits high tensile strength of more than 60 kg./mm. or yield stress of more than 45 kg./mm. and excellent coldworkability.

BACKGROUND OF INVENTION This invention relates to a novel nonthermal refining type high tension steel exhibiting excellent cold-workability, and more particularly such steel having high tensile strength or yield stress and exhibiting excellent coldworkability.

A high tension steel having tensile strength of 60 kg./ mm. 70 kg./mm. or 80 kg./mm. has already been developed and used. A number of steels has been developed which aims to improve cold-workability. However, no steel has been developed which exhibits both excellent cold-workability simultaneously with high tensile strength of 70 kg./mm. or 80 kg./mm. grade.

If a steel were developed exhibiting both high strength and excellent workability, it could be used in numerous instances, such as for use as a beam of a crane, a frame of autotrucks, a container material or a structural material for autos and other vehicles, etc. These uses require such properties as high strength, good cold-workability, good flatness, good weldability, high impact value and low cost.

For the purpose of satisfying the above requirements, known quenching and tempering treatments have usually been carried out to attain high tensile strength of 60 kg./ mm. 70 kg./mm. or 80 kg./mm. grades. However, such steel generally tends to become worse in cold-workability and flatness of surface in spite of improved workability of welding and impact strength. Also, disadvantageously, the cost is higher. Such tendency increases as the thickness increases. Thus, these steels are unsuitable for the above-mentioned purposes and uses. It is generally well known that cold-workability becomes worse as strength increases. In the prior art, there is no steel which exhibits excellent cold-workability and high strength and is available at low cost.

SUMMARY OF INVENTION 'Faced with the foregoing state of the art, the inventors carried out studies to improve weldability, impact strength and other properties and lower cost.

Accordingly, this inventive steel has been developed to 3,795,506 Patented Mar. 5, 1974 solve the above mentioned problems and eliminate the deficiencies of the prior art.

This invention encompasses a low C-Nb-Ti-Zr system composition. The chemical composition of this inventive steel substantially consists of, as the inventive portion thereof, 0.05 to 0.15% (by weight, the same holding herein) C and coexistence of 0.01 to 0.10% Nb, 0.03 to 0.15% Ti and 0.05 to 0.15 Zr. As other elements, 0.10 to 1.00% Si, 0.80 to 1.80% Mn and 0.01 to 0.10 sol. Al are included in steel. One or more elements selected from the group consisting of 0.10 to 0.50% Cr, 0.10 to 0.50% Cu, 0.10 to 0.50% M0, 003 to 0.20% V and 0.10 to 0.50% Ni can be further added. Unavoidable impurities may be present and the remainder is Fe. The inventive steel may be as rolled without any thermal refining treatment, and advantageously, has the excellent mechanical properties of high strength, good weldability, good coldworkability, good flatness, high impact strength, and low cost.

An object of the invention is to provide a non-thermal refining type high tension steel exhibiting excellent coldworkability simultaneously with high tensile strength.

Another object of this invention is to provide a nonthermal refining type high tension steel exhibiting good flatness of surface, good weldability and high impact value as well as excellent cold-workability and high strength as rolled without thermal treatments.

BRIEF DESCRIPTION OF DRAWING FIG. 1 depicts a graph showing change of impact properties with content of Ti or Zr;

FIG. 2 depicts a graph showing the relationship between content of Ti or Zr and notch elongation; and

FIG. 3 depicts a graph showing the relationship between tensile strength and notch elongation.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS The general effects of adding Nb, Ti or Zr to steel is qualitively known. Steels including Nb, Ti or Zr have been developed and used. For example, a typical steel is reported in Stahl and Eisen, (1970), Heft 22, S. 1255 to 1262 as Nb-Zr system steel or in Thyssenforschung, 2 (1970), Heft 3, S. 86 to as Ti-Nb system steel. However, when the above reports are examined in detail, it is found that the effects are far from the objects of this invention.

On the other hand, substantial coexistence of C, Nb, Ti and Zr in steel is an essential requirement of this invention. If any of these elements is lacking, the resulting steel cannot exhibit the desired properties, when rolled without any thermal refining treatment.

This invention encompasses steels having the following composition: C, 0.05 to 0.15%; Mn, 0.80 to 1.80%; Ti, 0.03 to 0.15%; sol. Al, 0.010 to 0.100%; Si, 0.10 to 1.00%; Nb, 0.01 to 0.10%; Zr, 0.05 to 0.15%; unavoidable impurities and remainder iron.

The above composition is preferred embodiment. When necessary, one or more elements selected from the following group of elements may be further added: Cr, 0.10 to 0.50%; Mo, 0.10 to 0.50%; Ni, 0.10 to 0.50%; Cu, 0.10 to 0.50% and V, 0.03 to 0.20%. By such addition, the mechanical properties are further improved, and it is pos- 3 sible to obtain desirable properties for relatively large thicknesses.

The reasons, which are fully documented in the following data, that content of each of the elements is limited as aforementioned are as follows:

As mentioned above, the substantial coexistence of Nb, Ti and Zr is one of the esential requirements. When one of the elements is not added, the required mechanical properties cannot be imparted to steel as rolled and without any thermal refining treatment. The properties are Less than 0.05% C does not give the required strength 5 based on geometrical action of the three elements. The to steel and more than 0.15% C makes cold-workability adding efiect of Ti or Zr among the three elements is worse. shown in FIGS. 1 and 2. Both FIGS. 1 and 2 show the Si plays a part in controlling strength in addition to further adding effects of Ti to Nb+Zr, including within producing a deoxidizing etfect. However, when Si content the range of the aforementioned amounts. Similarly, the is more than 1.00%, toughness of steel becomes Worse. further adding effects of Zr, to Nb-l-Ti are shown in FIGS.

In the case of less than 0.80% Mn, the required strength 1 and 2. is impossible to be obtained and when it is more than Referring to FIG. 1, it will be understood that the com- 1.80%, the formed crystal structure becomes unstable and position of elements specified in this invention is very eftoughness becomes worse. fective and the most suitable. When Ti content is more Nb is added to obtain the desired strength. However, in than 0.15%, both the fracture transition temperature and case of less than 0.01 the desired strength is impossible absorption energy at 0 C., which is measured in the transto be imparted to the steel and even if the Nb content is verse direction to rolling, become substantially worse. more than 1.80%, the adding elfect is saturated and con- When Zr content is less than 0.05 the fracture transisequently any greater percentage of Nb becomes meaningtion temperature is very high and the absorption energy less. is not improved. When Zr content is more than 0.15%,

Ti is added to improve the strength and simultaneously the absorption energy level becomes lower with the same therewith to play a part in adjusting formation of sulfide tendency as shown in the case of less than 0.05 in steel. The Of 1685 than ShQWS little In 2 changes of notch elongation substan. eifect and the addmg more tha11 015% raPldly tially corresponds to cold-workability, depending upon makes the cold-workability and impact properties worse. Ti or Zr content are Shown The relationship of Nb Ti 15 added to mamly, formmg f Sulfide and Zr content is in the same manner as that of FIG. 1. and improve cold-workability and 1mpactpropert1es.Howwhen Ti content is more than 0 15% the notch elon ever, in the case of less than 0.30% Zr, there is very little tion m i d1 become law Z 1 th 0 improvement of cold-workability and in the case of more th s b en 1 658 an than 0.15% Zr, conversely the impact properties become 6 9 6 onganon ecomes substantlany i worse, while the cold-workability is kept for the most part the T1 content ls the notFh elongatlon as it proved and shows little change even if the content is in- Sol. Al is necessary to deoxidi'ze and obtain good yield creasedof added Ti and Zr. Such effects are impossible to be ob- 3 Shows the l'elatlonshlp between tensile Strength tained in the case of less than 0.010% Sol. Al. More than and no ch elo Accordingly to the relationship 0.10% sol. Al makes fluidity and cleanliness of the batch shown in FIG. 3, it is apparent that this inventive steel in furnace. is far superior to the prior art steels.

In s invention, One more among group of The actual examples based on this invention are as ments comprising C11, M0, V and Ni y be further 40 follows. Such examples are shown in comparison with added to the above preferred composition, if necessarily prior art 1 or desired in order to obtain a desired level of strengt In such case, when content of each of Cr, Cu, Mo, Ni Manufactunng condltlons' is less than 0.10% 1' that Of 8 than 003%, the Chemical composition-As shown in Table 1 sired strength is not obtained. When the content of said Steel making stage MO and N Is than 93 or of Y Employed furnace-basic oxygen furnace, 85 ton giszznhgtggstfia, a disproportionate litt e etfect 15 obtained or After ordinary deoxidizing Operation kind of S content is not limited in this invention. However, it E Fe'Tl and Fe'Zr or d 1 b has been confirmed that less than 0.01% S is very eflecmnmg stage followed by ordinary castmg an 5 a tive for improving cold-workability. When tensile strength blngof more than kg./mm. is desired, the S content is a p y r011111; l-Tandern Hot Rolling Mill suitable means for Obtaining same. Hot rolling requnements-ordmary TABLE I C Si Mn P 8 Nb Ti Zr Sol-Al Others 0. eq.

631? 82 5 1 :1"? 8.0%? 8'80? 1 1919. 7. 3'0? 10 0.37 0.10 0.21 1.25 0. 010 0.010 0.000 0.18 0.08 0. 32 0.18 0. 23 1.27 0.011 0.008 0.035 0.08 0. 07 0. 40 0.12 0.20 0.85 0.018 0.020 0. 010 0.09 0.10 0.27 0. 00 0.82 1. 24 0.015 0.017 0.035 0.08 0.00 0. 31 0.11 0. 32 1. 20 0. 010 0.008 0. 035 0.12 0.07 0.33 0.10 0.28 1.22 0. 010 0.000 0. 040 0.00 0.08 0. s1 1- 205 2-20 2-20 0s 0s 2; 81 1 012% 1130 01010 01004 01038 0.10 0.13 0. 40 0.10 0.03 1. 40 0.015 0.008 0. 032 0.00 0. 07 M0 0. 08 0.50 1.08 0.018 0.005 0. 045 0.08 0.12 0.042 gggfgg 0.42 N--.:.:.'..: 0.10 0.38 1.22 0.015 0.010 0. 030 0.00 0.08 0 038 {1 ,12 2 0 Q33 1 Steels A to D are comparative steels. Steels E to N are this invention steels. 2 G.,eq.= 0+ (1/6)Mn+ /z4)Si+(1/40)N1+ (1/5)Or+ (1/4)Mo+ (1/14)V.

TAB LE II Impact properties 1 Tensile properties Cold bending workability Fracture Absorption Thick- Coiling Yield Tensile Notch transition energy at ness temperastress strength Bending 180 elongation tern era- (mm.) ture C.) (kg/mm!) (kg/mm!) (0.5 t.) (percent) ture C.) (kg.-II1.)

6. 610 58. 7 66. 6 Crack 2. 3 +100 0. 3 6. 0 640 52. 4 61. 8 d 3. 2 +40 0. 8 6. 0 590 64. 2 3. +30 1. 1 6. 0 510 75. 7 4. 4 --l0 2. 2 6. 0 525 58. 6 10. 2 -70 4. 4 6. 0 530 71. 3 5. 7 7 2. 2 6. 0 520 73. 4 6. 3 20 3. 0 8. 0 560 64. 4 8. 4 38 4. 0 8. 0 590 59. 4 7. 3 -32 3. 5 8. 0 590 68. 3 7. 2 -30 4. 3 8. 0 640 72. 8 6. 8 18 3. 1 10. 0 590 62. 3 8. 7 35 4. 0 10. 0 540 72. 5 6. 9 -28 3. 8 10. 0 560 59. 9 8. 9 65 4. 5

1 Sharp test piece: 5mm. x 10 mm. x 55 mm.

No'rE.Finishing thickness, 6.0 mm, to 10.0 mm.; Coiling requirements, ordinary. The mechanical properties of the above steels are least 60 kg./mm. consisting essentially of at least 0.05

shown in Table II.

According to Table I and Table II, it will be apparent that each of the strength (i.e. tensile strength and yield strength), the cold-workability (i.e. bending property and notch elongation) and the impact properties (i.e. fracture transition temperature and absorption energy) is far superior to that of the prior comparative steels. Tensile strength is in the 60 kg./mm. 70 kg./mm. or 80 kg./ mm. grades and yield stress is above 45 kg./mm. with relative ease and shows 60 lag/mm. and 75 kg./mm. range. Cold-workability is excellent. The carbon equivalent (C eq.) shown in Table I is low and shows good weldability. Simultaneously, it is also understood that the strength of steels including less than 0.01% S is superior to that of other steels. Moreover, production cost is low because among other reasons, the steel may be used as only rolled. There is no need for any thermal treatments. The strength may be further improved by adding one or more elements such as Cr, Cu, Mo, V or Ni.

The foregoing description is only illustrative of the principles of this invention. Numerous variations and modifications thereof would be apparent to one skilled in the art. All such variations and modifications are to be considered to be within the spirit and scope of the invention.

What is claimed is:

1. Nonthermal refiningtype high tension steel exhibiting excellent cold-workability and tensile strength of at to 0.15% C; 0.01 to 0.10% Nb; 0.03 to 0.15% Ti; 0.05 to 0.15% Zr, unavoidable impurities and remainder Fe.

2. Composition of claim 1, further comprising 0.10 to 1.00% Si; 0.80 to 1.80% Mn and 0.01 to 0.10% sol. Al.

3. Composition of claim 2, further comprising one or more of the elements selected from the group consisting of 0.10 to 0.50% Cr; 0.10 to 0.50% Cu; 0.10 to 0.50% Mo; 0.03 to 0.20% V and 0.10 to 0.50% Ni.

4. Composition of claim 3, further comprising S in an amount less than 0.010%.

References Cited UNITED STATES PATENTS -123 1, 123 M, 126 D 128 T Wagner 7 5-124 

